THE DEVELOPMENT OF THE VASCULAR SYSTEM. 289 



A left renal vein' may open into the left common iliac, which condition 

 represents a persistence of the more caudal part of the left posterior cardinal 

 (Fig. 236). This anomaly is rare. 



The azygos vein occasionally presents variations which are due to anoma- 

 lous development. All the intercostal veins on the left side may be collected 

 into a vessel which opens into the left innominate vein. There may be a single 

 median azygos vein; or there may be a transposition of the azygos vein. It may 

 be on the left side and open into the coronary sinus (normal conditions in the 

 sheep and a few other Mammals) . The latter condition represents a persistence 

 of the more cephalic part of the left posterior cardinal vein (see Figs. 233 and 234) . 



Space does not permit a discussion of the great number of congenital varia- 

 tions that occur in the smaller blood vessels, both arteries and veins. The 

 student is referred, however, to the more extensive text-books of anatomy. 



PRACTICAL SUGGESTIONS. 



The development of the vascular system (blood vessels, lymphatic vessels and blood) 

 presents some of the greatest difficulties of study in embryology. The development of the 

 blood cells and the formation of the primary blood vessels may be studied by means of 

 ordinary histological technic. But in order to trace the development of such complicated 

 systems as the blood vessels and lymphatic vessels, other methods, in addition to ordinary 

 histological technic, must be employed; for it is obvious that a few sections taken at random 

 in an embryo would be practically valueless in tracing the course of a vessel. These addi- 

 tional methods, as referred to in the following paragraphs, will be described in the Appendix. 



The Blood. — The blood islands are very well shown in surface views of the chick blasto- 

 derm. The most instructive specimens are obtained during the latter part of the first and 

 during the second day of incubation. The blastoderm is removed from the egg, fixed in 

 Zenker's fluid, stained in loto with borax-carmin and mounted in toto in xylol-damar. 



To complete the study, sections of blastoderms of similar stages are also necessary. In 

 this case Flemming's fluid is an excellent fixative (Zenker's is good, but causes more shrink- 

 age). Sections are cut in paraffin, stained with Heidenhain's haematoxylin and mounted 

 in xylol-damar. 



In later stages the blood cells may be observed in the vessels in sections of any embryo (see 

 following paragraphs). The best region is the liver, where the cells are always present in 

 great numbers. 



Very instructive specimens may be obtained by making smears, at any stage during 

 embryonic life, from a fresh liver or spleen, or from the bone marrow, allowing the smear 

 to dry, staining with Jenner's blood stain and mounting in xylol-damar. In such specimens 

 all types of blood cells may be seen. 



The Blood Vessels. — Surface view of the chick blastoderm during the second and third 

 (and even later) days of incubation show the developing blood vessels in the extraembryonic 

 area, and also show the relation between the vessels and blood islands. The blastoderm 

 is removed from the egg, fixed in Zenker's fluid, stained in toto with borax-carmin and 

 mounted in toto in xylol-damar. 



Sections of the blastoderm at such stages are necessary to complete the picture one gets 

 on surface view. The blastoderm is fixed in Flemming's fluid or Zenker's fluid, sectioned 

 in paraffin, stained with Heidenhain's haematoxylin and mounted in xylol-damar. The for- 



